The goals of proposed experiments are to improve the continuity of the partially completed physical map of the C. elegans genome; to refine the projection of the genetic map onto the physical map; and to develop applications of the map. At present the map consists of 700 "contigs" formed by overlapping cosmids, with an average size of 100 kb. Only one hundred of these are associated with genetically positioned sequences. To recover the sequences that span the 700 gaps between the contigs, a library of large genomic fragments cloned into a yeast artificial chromosome (YAC) vector will be constructed. The library will be screened by hybridization for clones that overlap with the end-clones of each contig. These YAC clones will then be hybridized to an array of cosmid clones that represent the presently cloned genome to recognize contigs that are adjacent. Preliminary experiments show that this strategy is powerful and should lead to rapid closure of a large portion of the map. Additional efforts will then be required to recover the remaining gap sequences, or to link contigs across the gap by macrorestriction fragments. Additional points of correspondence between the physical and genetic maps will be established through transformation rescue of known mutants with specific fragments, either as cosmid or YAC clones, from the physical map. Other applications of the physical map will be explored as time permits. The parameters of the YAC cloning system that are established in our work will be of relevant to those contemplating the use of this system in more complex mapping projects. The overall approaches needed to complete the physical map will also be of general interest to those planning similar projects in more complex genomes. And the ways in which the physical map can be applied in C. elegans should stimulate efforts in other projects too. Finally the map will provide a rapid means of recovering any gene in C. elegans and will act as a starting point for studying large-scale genome organization.